Apparent ecosystem carbon turnover time: uncertainties and robust features

Fan, Naixin; Koirala, Sujan; Reichstein, Markus; Thurner, Martin; Avitabile, Valerio; Santoro, Maurizio; Ahrens, Bernhard; Weber, Ulrich; Carvalhais, Nuno

The turnover time of terrestrial ecosystem carbon is an emergent ecosystem property that quantifies the strength of land surface on the global carbon cycle–climate feedback. However, observation- and modeling-based estimates of carbon turnover and its response to climate are still characterized by large uncertainties. In this study, by assessing the apparent whole ecosystem carbon turnover times (inline-formulaτ) as the ratio between carbon stocks and fluxes, we provide an update of this ecosystem level diagnostic and its associated uncertainties in high spatial resolution (0.083inline-formula) using multiple, state-of-the-art, observation-based datasets of soil organic carbon stock (inline-formulaCsoil), vegetation biomass (inline-formulaCveg) and gross primary productivity (GPP). Using this new ensemble of data, we estimated the global median inline-formulaτ to be inline-formula M6inlinescrollmathml normal 43 - normal 7 + normal 7 24pt17ptsvg-formulamathimga964b3b30c1860cbfb56e35448624327 essd-12-2517-2020-ie00001.svg24pt17ptessd-12-2517-2020-ie00001.png inline-formulayr (inline-formula M8inlinescrollmathml median - difference to percentile 25 + difference to percentile 75 135pt20ptsvg-formulamathimg8a54348845f175bc742186629122fb63 essd-12-2517-2020-ie00002.svg135pt20ptessd-12-2517-2020-ie00002.png ) when the full soil is considered, in contrast to limiting it to 1 inline-formulam depth. Only considering the top 1 inline-formulam of soil carbon in circumpolar regions (assuming maximum active layer depth is up to 1 inline-formulam) yields a global median inline-formulaτ of inline-formula M13inlinescrollmathml normal 37 - normal 6 + normal 3 24pt17ptsvg-formulamathimg066305b928331c7b879dc48e187744a3 essd-12-2517-2020-ie00003.svg24pt17ptessd-12-2517-2020-ie00003.png inline-formulayr, which is longer than the previous estimates of inline-formula M15inlinescrollmathml normal 23 - normal 4 + normal 7 24pt17ptsvg-formulamathimg9e1c1f4bd0e302dbebbbe024a3cf245f essd-12-2517-2020-ie00004.svg24pt17ptessd-12-2517-2020-ie00004.png inline-formulayr (Carvalhais et al., 2014). We show that the difference is mostly attributed to changes in global inline-formulaCsoil estimates. inline-formulaCsoil accounts for approximately 84 % of the total uncertainty in global inline-formulaτ estimates; GPP also contributes significantly (15 %), whereas inline-formulaCveg contributes only marginally (less than 1 %) to the total uncertainty. The high uncertainty in inline-formulaCsoil is reflected in the large range across state-of-the-art data products, in which full-depth inline-formulaCsoil spans between 3362 and 4792 inline-formulaPgC. The uncertainty is especially high in circumpolar regions with an uncertainty of 50 % and a low spatial correlation between the different datasets (inline-formula M24inlinescrollmathml normal 0.2 < r < normal 0.5 63pt10ptsvg-formulamathimg0c74f3c5f44fa4a72ab00f103017d905 essd-12-2517-2020-ie00005.svg63pt10ptessd-12-2517-2020-ie00005.png ) when compared to other regions (inline-formula M25inlinescrollmathml normal 0.6 < r < normal 0.8 63pt10ptsvg-formulamathimg0c48fe27e45d5730a498149c8387e90b essd-12-2517-2020-ie00006.svg63pt10ptessd-12-2517-2020-ie00006.png ). These uncertainties cast a shadow on current global estimates of inline-formulaτ in circumpolar regions, for which further geographical representativeness and clarification on variations in inline-formulaCsoil with soil depth are needed. Different GPP estimates contribute significantly to the uncertainties of inline-formulaτ mainly in semiarid and arid regions, whereas inline-formulaCveg causes the uncertainties of inline-formulaτ in the subtropics and tropics. In spite of the large uncertainties, our findings reveal that the latitudinal gradients of inline-formulaτ are consistent across different datasets and soil depths. The current results show a strong ensemble agreement on the negative correlation between inline-formulaτ and temperature along latitude that is stronger in temperate zones (30–60inline-formula N) than in the subtropical and tropical zones (30inline-formula S–30inline-formula N). Additionally, while the strength of the inline-formulaτ–precipitation correlation was dependent on the inline-formulaCsoil data source, the latitudinal gradients also agree among different ensemble members. Overall, and despite the large variation in inline-formulaτ, we identified robust features in the spatial patterns of inline-formulaτ that emerge beyond the differences stemming from the data-driven estimates of inline-formulaCsoil, inline-formulaCveg and GPP. These robust patterns, and associated uncertainties, can be used to infer inline-formulaτ–climate relationships and for constraining contemporaneous behavior ofpage2518 Earth system models (ESMs), which could contribute to uncertainty reductions in future projections of the carbon cycle–climate feedback. The dataset of inline-formulaτ is openly available at https://doi.org/10.17871/bgitau.201911https://doi.org/10.17871/bgitau.201911 (Fan et al., 2019).

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Fan, Naixin / Koirala, Sujan / Reichstein, Markus / et al: Apparent ecosystem carbon turnover time: uncertainties and robust features. 2020. Copernicus Publications.

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